Electronic scanning of corporate structure antennas has been greatly simplified in the number of power dividing and phasing matrices required by the Butler matrix which utilizes the phase shifts occurring in hybrid dividers. The theory, construction and operation of the Butler matrix is well known and is explained in detail in the article "Beam-Forming Matrix Simplifies Design of Electronically Scanned Antennas" by Jesse Butler and Ralph Lowe, Electronic Design, Apr. 12, 1961. Although the Butler system is effective for accomplishing beam steering it is incapable of minimizing the effects of undesired radiation.
The problem of minimizing undesirable received signals has been approached by the use of "adaptive arrays." The design of an adaptive array is dependent upon the principles of feedback design. The main objective of the array is to minimize an undesired signal or to maximize the desired signal in a given direction. Typically, a broad beam is formed by using a small number of elements. When an undesired signal is incident on the antenna, it is split into in-phase and quadrature components, compared with a reference signal and integrated. If no correlation is achieved, the weights or excitation coefficients, W, in each of the in-phase and quadrature channels are adjusted to place a minimum in the direction of the undesired signal. Adaptive antenna systems are further described in the article by Robert L. Riegler and Ralph T. Compton, Jr., "An Adaptive Array for Interference Rejection," Proc. IEEE, Volume 61, No. 6, June 1973 and also in the article "Adaptive Antenna Systems," by B. Widrow, P. E. Mantey, L. J. Griffiths, and B. B. Goode, Proc. IEEE, Volume 55, No. 12, December 1967, both articles incorporated herein by reference. The disadvantage of the adaptive systems heretofore described is basically that the systems are receive only and, thereby, require a separate system for transmission.